This invention relates to positive photoresist compositions which contain radiation-absorbing compounds as additives for suppressing stray radiation or halation effects.
Photoresists serve as versatile high-grade process chemicals in the manufacture of electric circuits and electronic components and are used in particular in the structuring of conductor and semiconductor substrates by photolithographic techniques.
In the process sequences customarily used, the substrates are coated with the photoresist composition. The photoresist layer is then exposed through an image original to high-energy radiation, generally ultraviolet radiation, and thereafter developed.
In the case of positive photoresists, those areas of the layer which, in correspondence with the image pattern of the original, are exposed to radiation become more soluble in a developer through the action of the radiation. In the developing step, the exposed parts of the layer are removed, leaving behind the unexposed and thus insoluble parts as a relief structure which represents a positive image of the original on the substrate surface.
In the case of negative photoresists the effect of the radiation is to crosslink the layer in the exposed regions which reduces the solubility in a corresponding developer. In the developing step, the unirradiated areas of the layer are removed. A negative image of the original remains behind as a relief structure on the substrate.
The ever advancing miniaturization in semiconductor technology and microelectronics places ever higher demands on the image structures to be represented by means of photoresist materials. This applies particularly to the resolution and edge steepness of the photoresist relief structures, but also to adhesion, mechanical and chemical stability, dimensional fidelity, resistance at elevated temperatures and to other factors which can play a part in the subsequent process steps.
Positive photoresists are today still preferred in the production of fine and very fine structures, since they have the better properties, in particular with respect to resolving power. The positive photoresist compositions customarily used today essentially contain, dissolved in an organic solvent, phenol-formaldehyde condensates of the novolak resin type and compounds of the type of o-diazoquinones as light-sensitive components. Such compositions can be developed in aqueous alkaline developing baths.
However, state of the art positive photoresists are still capable of improvement. It is generally a known problem in photoresist technology that in applications to substrates which have a surface of high reflective power, which is generally the case with semiconductor materials and metals such as, for example, silicon, aluminum, copper, chrome, etc., the resolving power of the photoresist is appreciably reduced by reflections occurring at the surface of the substrate. As a result of irregular scattering of the incident light at the substrate surface, stray light penetrates into areas of the resist layer which, according to the image pattern of the original are not supposed to be exposed to any radiation. Such scattering and halation effects give rise to relief structures having reduced edge crispness and steepness after development. Fine and very fine structures, e.g., as needed in microelectronics, are obtainable only with limited fidelity of reproduction, as a consequence of these effects.
To overcome such problems it is known, in particular from negative photoresist technology, to add to the resist compositions radiation-absorbing dyes to reduce undesirable exposure effects. The demands placed on such dyes are, for example, high absorption power within the wavelength range of the exposure apparatus, compatibility with the photoresist composition, e.g., in particular a sufficient solubility therein, and stability under the conditions of the process sequences.
Of the dyes used hitherto for such purposes, aminoazobenzene derivatives, as proposed for example in Japanese Patent 51 (76) 37,562 or U.S. Pat. No. 4,268,603 as additives to negative photoresists based on cyclized rubber, and bisazido radiation crosslinkers have the most favorable absorption properties. These substances, however, have the disadvantage of being only relatively sparingly soluble in photoresists and, moreover, subliming out of the resist layer to an appreciable degree at elevated temperatures up to about 100.degree. C. The latter are customarily employed in the process step of predrying or prebaking the applied coating. More favorable in this respect are the aminoazobenzene derivatives of German Offenlegungsschrift 3,324,795 (U.S. application Ser. No. 628,881 of Jul. 9, 1984), which are still stable at temperatures around 120.degree. C.
It has been found, however, that despite their favorable absorption properties, the use of the negative photoresist technology dyes in positive photoresists does not lead to the desired success. The process sequences for structuring semiconductor substrates by means of positive photoresists frequently give rise to temperatures of 150.degree. to about 200.degree. C., for example in the customary heat treatment steps of prebaking and postbaking which are necessary for increasing the adhesion of the resist layer to the substrate and for increasing its stability. Moreover, such thermal stresses occur in the course of plasma etching and ion implantation processes. Even the most heat-stable dyes known at these temperatures sublime out of the resist layer. As a result, the resist layer loses the desired absorption properties. This then has a drastic negative effect on the resolving power and edge steepness of the resist image to be produced. At the same time the sublimed dye causes extremely undesirable contamination in processing equipment.